23(3 THE AGE OF THE EARTH AS AN ABODE FITTED FOR LIFE. 



ern scientific probability. It also applies to any hypothesis which 

 postulates aggregation from a dispersed condition. In any case, it 

 seems necessary to determine when the moon be^^ame full grown 

 before it is possible to assign a positive date for the commencement 

 of effective tidal action. It would appear that such action might be 

 developed gradually, as the material of the moon became aggregated. 

 Dui-ing such gradual assumption of the tidal function the reaction 

 between the moon and the earth must have been of a feeble sort, and 

 a recomputation of its amount, based on a series of hvpotheses which 

 shall cover the whole ground of legitimate speculation, would seem 

 necessary before any satisfactory conclusions can be reached. 



It maybe urged that the computations of George Darwin, following, 

 in 1)ackward steps, by the masterly application of mathematical analysis, 

 the stages of the earth-moon relationship, give a firmer ground for 

 conclusions. In a qualified degree this must be conceded. But it is 

 to be remarked, in the first place, that the mathematics becomes inde- 

 cisive before the origin of the moon is reached, which may signify 

 that this is not the true line of approach to the origin of the moon, or 

 that there is some error or defect in the assumptions. It would seem 

 to be obvious, however, that if the tidal function was the result of a 

 slow aggregation which began at an indeterminate stage in the earth's 

 existence the numerical results of a computation based on a full- 

 grown moon may need radical revision. 



Furthermore, the agencies which are assumed to have accelerated 

 the rotation of the earth in its earlier history must not be neglected. 

 If they may safely be assumed to have been competent to give the 

 earth a rotary speed sufficient to detach from itself the matter of the 

 moon, as is postulated in the Laplacean and the fission hypotheses in 

 common, the same agencies, if more evenly distributed in time, might 

 prolong the period of acceleration so that it should be coincident with 

 that of tidal retardation and offset it in any degree that falls within 

 the legitimate limits of assumption. We encounter here again, in 

 another form, a deduction from the assumption of a ver}^ rapid con- 

 centration of the matter ingathered to form the earth and moon, and 

 the consequent exhaustion of its energy in an early stage. If, how- 

 ever, the concentration were less rapid and less complete in the early 

 history of the earth, as is postulated by the accretion theory, as herein 

 entertained, acceleration might be far less advanced in the earliest 

 stages and be greater in the later stages. Hence, the retarding effects 

 of tidal friction may have been more effectually antagonized by the 

 shrinkage of the earth during the progress of geological history. Mr. 

 Moulton has computed the effects of the internal change of metal and 

 rock material, assumed in a hypothetical case on a previous page, on 

 the speed of rotation of the earth, and found that it would accelerate 

 the then current rate, whatever it was, about one-fifth. If, therefore, 



